1. Field of the Invention
The present invention relates to a method for driving an optical write device, and more particularly to a driving method which enables an optical write device to write multi-tone images (latent images) on a photosensitive member.
2. Description of Related Art
Generally, for optical write devices for forming images (latent images) on a silver-salt film or an electrophotographic photosensitive member, it is necessary to drive a light shutter array made of PLZT or the like in multiple tone levels of eight to twelve bits (256 to 4096 levels) for the purpose of reproducing high-quality full color images. Conventionally, it has been practiced that the pulse width or the pulse intensity of a drive signal is modulated in accordance with image data of a plurality of bits. By employing such control, the conventional devices represented tone of a prescribed pixel by one dot. However, only with the modulation control based on image data, driver ICs beyond six bits for exclusive use are necessary for the high-quality multi-tone reproduction, and manufacturing such driver ICs wold result in much higher costs.
An object of the present invention is to provide a method for driving an optical write device by which the device can employ a structure of low cost.
The other object of the present invention is to provide a method for driving an optical write device which, with low-bit driver ICs for general use, enables the optical write device to reproduce multi-tone images of more bits.
Still further object of the present invention is to provide a method for driving an optical write device which allows the selection of a type of reproduction from different types (e.g., reproduction with smooth density gradation and reproduction with multi-line screen processing) in addition to the above object.
In order to attain the objects above, in a method for driving an optical write device according to the present invention, a time for formation of a pixel is divided into a plurality of periods, and in each of the periods, tone level control based on image data is performed. For example, a case in which six-bit image data for 64 tone levels are given and the time for formation of a pixel is divided into three periods for eight tone levels is described. In this case, the image data are transferred three times for formation of a pixel, and 2(3+6)xe2x88x92(26xe2x88x921)xe2x88x92(23xe2x88x921)=442 tone levels can be reproduced. In a device adopting the method of this example, if an image to be reproduced is given as eight-bit image data (256 tone levels), the eight-bit (256-level) image data are converted into data for 442 tone levels (for example, referring to a look-up table), and sets of image data for the respective periods are outputted. Because of the reproducibility of 442 tone levels, tone level correction such and xcex3 correction which may be required to make the image match the human""s visual sensitivity and the sensitivity of the photosensitive member can be carried out. It also becomes possible to attain smoother representation by performing interpolation among data on the tone levels of pixels.
Therefore, according to the present invention, with six-bit driver ICs for general use, multi-tone images of more bits can be reproduced.
In the present invention, the tone level control based on image data may be either modulation in pulse width or modulation in pulse intensity. The time for formation of a pixel is divided into a plurality of periods either by a train of pulses with widths weighted by 2xe2x80x3 or by a train of pulses with uniform widths. If the multi-tone reproduction by the pulses is assigned to lower-order bits, image reproduction with smooth density gradation can be attained, which is suitable for writing on silver-salt films. On the other hand, if the multi-tone reproduction by the pulses is assigned to high-order bits, image reproduction with multi-line screen processing can be attained, which is suitable for writing on electrophotographic photosensitive members. Further, if the pulses are allocated in a discrete manner, image reproduction with smooth density gradation can be attained, and if the pulses are allocated in a concentrated manner, image reproduction with multi-line screen processing can be attained.